Yes, it's normal for it to change. Sometimes it will be lower because they're sleeping. For example, I was getting 160ish heartbeats early on, and then at my 20 week u/s, it was around 130. He was sound asleep and slept through the entire u/s!
Next visit, he was moving around and was back up in the 150s.
The heartbeat also slows down as they get closer to your due date anyway. At birth, I don't think they usually keep the 150-160 heartrate.
My last pregnancy, DS's heartrate was in the 150s throughout, and then when I was getting ready to deliver (preterm due to PPROM), I was on a monitor for 4 days, and his heartrate stayed right around 135 the entire time. Then when he was born, his heartrate was still the same the entire time he was in the NICU! It was kind of neat.
But anyway, no worries about heartrates changing. That is perfectly normal.